A memory device includes a programming transistor and a reading transistor of an anti-fuse memory cell. The programming transistor includes first semiconductor nanostructures vertically spaced apart from one another, each of the first semiconductor nanostructures having a first width along a first lateral direction. The reading transistor includes second semiconductor nanostructures vertically spaced apart from one another, each of the second semiconductor nanostructures having a second width different from the first width along the second direction. The memory device also includes a first and a second gate metals. The first gate metal wraps around each of the first semiconductor nanostructures with a first gate dielectric disposed therein. The second gate metal wraps around each of the second semiconductor nanostructures with a second gate dielectric disposed therein.